1. Field of the Invention
The present invention relates to a system for attenuating noise electronically and, in particular, to an electronic noise attenuation system which is capable of attenuating non-steady noise occurring in propagation passages such as duct lines or the like by exercising an adaptive control using a computer system including a digital filter therein.
2. Description of the Prior Art
Conventionally, there has been widely put into practical use a passive noise attenuation apparatus which attenuates noise occurring within ducts by use of the interference due to the duct structure or the noise absorption due to a porous material attached to the duct. However, this type of noise attenuation apparatus is found disadvantageous in that it is too big in size, it involves too much loss of pressure, and so on.
On the other hand, there is also available an active noise attenuation apparatus which has been long proposed and employs another method of the reduction of unwanted sounds within the duct. That is, recently, special interest has been given to an electronic noise attenuation system of such active type in which noise propagated from a source of noise is sensed, a cancellation sound having the same sound pressure and an opposite phase with respect to the sensed noise is generated against the noise to thereby provide sound wave interference between the noise and the cancellation sound, and thus the noise can be cancelled forcibly by the sound wave interference. With the rapid progress of an electronic device, signal processing technique and the like, there have been recently published various kinds of study results on such active electronic noise attenuation method and apparatus.
However, there are still left many problems to be solved and thus such electronic noise attenuation method or apparatus has not yet come into a stage of seriously practical application.
A technical problem in putting into practice such electronic noise attenuation system consists in the construction of a model which can be used as a basis for design of a control system of the electronic noise attenuation system. The model must be able to cope with the following points. At first, there is necessary a filter which is capable of cancelling noise of continuous spectra. That is, if a cancellation sound can be generated with respect to the noise of continuous spectra such as automotive noise, air current noise and the like as well as the noise of discrete spectra such as transformer noise, compressor noise and the like, the applications of the electronic noise attenuation system can then be expanded further. To realize this, a filter is required which is able to provide arbitrary amplitude characteristics and phase characteristics.
Secondly, it is necessary to prevent the feedback of the cancellation sound with respect to a sensing microphone. That is, in the electronic noise attenuation system, there is interposed the sensing microphone between a source of noise and a source of cancellation sounds within a propagation passage through which sound waves are propagated, and it is necessary to create an electric signal to drive the cancellation sound source which generates sound waves to cancel the propagated sound waves from the noise source, in accordance with the sounds sensed by the sensing microphone and by some proper signal generation means. In this case, the sound waves generated from the cancellation sound source is also caught by the sensing microphone and, as result of this, there is produced an acoustic feedback system between the cancellation sound source and the sensing microphone. For this reason, it is essential to take a countermeasure to cope with this situation. Especially in order to make compact the electronic noise attenuation system and to allow it to be mounted at an arbitrary position in a pipe line such as a duct line, the sensing microphone and the cancellation sound source must be located adjacent to each other. Therefore, the above-mentioned acoustic feedback has a great influence on the electronic noise attenuation system and thus the countermeasure to cope with this problem is very important.
Thirdly, it is necessary to make it possible to correct the characteristics of electro-acoustic transducers such as a microphone, speaker and the like used in the electronic noise attenuation system. That is, in order to stabilize the control function of the electronic noise attenuation system, it is essential that the control system of the electronic noise attenuation system is provided with a function to correct the minute amount of deterioration of the characteristics of the electro-acoustic transducers. This is another problem to be solved.
In view of this, we have already found and proposed models for an electronic noise attenuation system which can cope with the above-mentioned problems (Japanese Patent Application No.60-139293, No.60-139294, No.61-7115, No.62-148254.)
According to the electronic noise attenuation system that we have proposed, the above-mentioned third problem can be solved properly: that is, by properly controlling the characteristics of a digital filter for creating an electric signal to be given to a cancellation sound source, the system can cope with the variations of the propagation characteristics of a sound wave propagation passage (e.g., a duct) as well as the variations of the characteristics of a control system (which includes a speaker as a cancellation sound source, a microphone as a sensor and the like).
Referring now to FIG. 1, there is shown a basic structure of a monopole sound source type of adaptive electronic noise attenuation system including two sensor microphones M1, M2.
In this structure, the output of the sensor microphone M2, which is located on the down stream side of the figure, is as an error signal. The basic operation of the structure is to update the transfer function of a digital filter 2 from the input X of the digital filter 2 and the output E of the sensor microphone M2 so that the energy of the output E can be a minimum value under some evaluation standard or other.
Now, if an actual electronic noise attenuation system is modeled according to FIG. 1, then a model shown in FIG. 2 can be obtained. The model shown in FIG. 2 is constructed on the assumption that a sound wave to be fed back from a cancellation sound speaker (an additional sound source) S to the sensor microphone M1 is cancelled electrically at a point of addition 20 and thus it is not input to the digital filter 2.
What is important here is the existence of a transfer function D with a time delay representing the transfer characteristics of speaker, duct and the like from the output of the digital filter 2 to the addition point of the error signal.
By the way, in order to be able to apply a well-known adaptive control algorithm such as VS-LMS (Variable Step-Least Means Square) or the like, not only the input X of an adaptive digital filter must be defined clearly but also it is necessary to clarify the connection of the output Y of the digital filter with an error signal E. In the case of a system in which after the output of the digital filter 2 is determined the error signal E can be observed in an instant or a system in which the error signal E has already been decided at latest by the time of updating of the next coefficient of the digital filter, basically there arises no problem and thus the well-known algorithm can be applied. An echo canceller filter is a good example to deal with an acoustic signal and in this filter the output Y of the filter is reflected, as it is, in the error signal E. In contrast to this, in the electronic noise attenuation system shown in FIG. 1, the film output is not connected, as it is, with the error signal E but the error signal E can be obtained only by means of the electro-acoustic conversion characteristics of speaker, transfer characteristics from speaker to microphone, process of super-position (interference) of acoustic signals in space, and the acoustic-electric conversion characteristics of microphone. That is, if the above-mentioned transfer function D is not taken into consideration, then a sound cancellation effect cannot be obtained at all.
Further, in our previous application for patent (Japanese Patent Application No. 62-148254), as shown in FIG. 8, the restriction of an acoustic feedback is effective only when the transfer function from the speaker S to the microphone M1 is practically equal to that from the speaker S to the microphone M2. Most of linear duct equipment can satisfy this requirement.
However, when a sound cancelling device is constructed by mounting speaker to the bent portion of a duct, the above-mentioned structure is not able to perform its function to the full. For this reason, the present invention is proposed. Since the restriction of the acoustic feedback is performed by means of identification of the transfer function of a feedback system, the invention can be applied to any duct whatever shape it has. Also, the invention can apply even to an active sound cancellation system in a three-dimensional sound field (outdoor or indoor).